Ring type debarkers of the type commonly used in the lumber and paper industry to remove bark from logs, have included a triangular configuration of infeed and outfeed rollers to apply thrust to a log to force it longitudinally through a rotating ring debarking unit while holding the log to prevent rotation of the log while bark is being removed.
In the prior art apparatus, as a log enters the roller assembly, the rollers are held in a closed position by a preset, pretension force on the rollers, and during initial engagement of a leading end of a log against the rollers when the rollers are in their closed position, substantial force is required to cause the infeed rollers to climb the leading end of the log so that the rollers can then ride on the outside surface of the logs and apply a compressive force against the logs. During the stage of the operation wherein the leading end of a log first contacts the rollers, and as the rollers separate to permit the log to move between the rollers, and prior to positioning of the rollers on the exterior surface of the log, the tension force on the rollers serves no purpose other than as a deterrent to roll opening and movement of the log through the debarking machine. Additionally, as the rolls begin to climb the leading end of the log, the holddown force on the rollers increases as the rollers move outwardly. If the log engaging the rollers has a large cross sectional diameter, a substantial amount of power is required to cause the rollers to climb the leading end of the log. In some cases, the increase in holddown force caused by initial outward movement of the rollers, as the leading end of the log engages the rollers, may be so great that the rollers will not be permitted to move to the open, log engaging position. The operator must then manually cause the rollers to open by means of additional valving.
In the existing machines, a pneumatic cylinder is used to apply a preset holddown force on the rollers, and a certain pressure, typically 50 to 70 psi, is applied to the rod side of the pneumatic cylinder causing it to retract, and, in so doing, to cause the feed rollers to close. A pressure regulator is used to control this pressure in the pneumatic cylinder, and a check valve is typically used downstream of the pressure regulator. The check valve insures that backflow cannot occur. A relief valve is teed in the line between the check valve and the rod side of the cylinder. Typically this relief valve is set at 90 psi. On some installments the blank side or cap end of the cylinder is connected to another pressure regulator set at a pressure of 5 to 20 psi. This fluid line also has a check valve and a relief valve, the relief valve being set typically at 45 psi.
During normal operation of the prior art structures, when the rollers are in the extended or open position, the pressure regulator connected to the blank side of the pneumatic cylinder applies a 5 to 20 psi pressure to the blank side volume. When the logs passes through the rollers, the pressure in the rod side of the cylinder causes the rollers to close, and the closing of the rollers is opposed by the 5 to 20 psi pressure in the blank end of the cylinder. This pressure will increase until the relief valve setting of 45 psi is reached whereupon the relief valve will open and maintain a 45 psi back pressure. Should the relief valve be undersized or in some way become restricted, the pressure in the blank side of the pneumatic cylinder can climb to a higher level. This will cause a net reduction in the holddown force and a slowing down in the closing action of the rollers. As the log clears the infeed rollers, it is preferred that the rollers close rapidly in anticipation of the next log. Should the next log have a small diameter at its leading end relative to the diameter of the trailing end of the previous log, slow closing of the rollers may result in the second log contacting the debarking tools prior to engagement of the rollers with the log. When this occurs, the log can flail and/or move out of the tool circle, either of which causes premature tool breakage, link bracket breakage and bearing failure.
Attention is also directed to the Brundell et al. U.S. Pat. No. 2,903,028, issued Sept. 18, 1959: the Brundell et al. U.S. Pat. No. 2,785,715, issued Mar. 19, 1957; the Brundell et al. U.S. Pat. No. 2,857,945, issued Oct. 28, 1958 and the Brundell et al. U.S. Pat. No. 2,866,672, issued Nov. 28, 1958.